The qualitative and quantitative determination of specific binding agents and of the corresponding binding partners thereof are methods used widely in the biological sciences. The methods rely on tagging or labeling either the binding agent or binding partner or both so that presence of the ligands is traceable. Examples of the above-noted agent-partner reaction are antibody-antigen, antigen-antibody, protein-protein, ligand-chelate, ligand-protein, protein-ligand, ligand-receptor, receptor-ligand, nucleic acid-complementary nucleic acid, oligopeptide-protein, protein-oligopeptide and the like.
In recent years, small size metal particles or precipitates of metal salts such as colloidal gold particles have gained popularity as markers for the localization, characterization, and quantitation of specific organic substances and structures. The detection of biological materials by metal labeling has proved useful in cytology and histology.